Process for preparing fluoroolefins



United States Patent PROCESS FOR PREPARING FLUOROOLEFINS FROM FLUOROCYCLOBUTANES John Lynde Anderson, Northwood, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 29, 1954, Serial No. 465,747

Claims. (Cl. 260-653) This invention relates to organic compounds and more particularly to fluorine containing organic compounds.

This invention has as an object the preparation of monomeric intermediates for. polymers. Another object is the preparation of intermediates for chemical syntheses. Other objects will appear hereinafter.

These objects are accomplished by the present invention wherein difluoroolefins are obtained by pyrolyzing, at 600-1000 C., a tetrafluorocyclobutane having two adjacent annular CFz groups, and an annular CH2 group, any substituent or substituents on the fourth annular carbon being hydrocarbon and attached by from one to' two bonds to said fourth annular carbon, said hydrocarbon radicals being, within the radical, free from non-aromatic unsaturation, any remaining valence of the fourth annular carbon being bonded to hydrogen, said tetrafluorocyclobutane being of from four to seventeen carbons.

' The course of this reaction is illustrated by the follow- Q ing equation:

Flo-on. 1 20:0112 FgC=O- -R or heat . 1'. wherein R and R are hydrogen, a monovalent hydrocarbon radical free from non-aromatic unsaturation of up to eight carbon atoms, whether alkyl, cycloalkyl, aralkyl, or aryl, or R and R together forming a divalent hydrocarbon radical Whose two free valences are attached to the same carbon, the total carbons in R and R being no more than thirteen. As the above equation indicates, the cyclobutane ring can be cleaved in either of two ways. Under some conditions cleavage takes place predominantly in but one of these ways with minor amounts of cleavage in the other way, while under other conditions both types of cleavage take place in not widely differing amounts. The

particular hydrocarbon substituent on the annular carbon atom influences the type of cleavage taking place. In both types of cleavage, however, 1,1-difiuoroolefins, having only two fluorine atoms, are always produced. One of the products of this process, 1,1-difluoroallene, CF2=C=CH2, is a new compound and it is a part of this invention.

The process of this invention is preferably carried out by passing the 1,l,2,2-tetrafluorocyclobutane through a reaction zone heated to a temperature between 750 and 900 C.

The pressure at which the pyrolysis of the 1,1,2,2-tetrafluorocyclobutane is carried out is not critical, pressures ranging from a few microns of mercury to atmospheric or even superatmospheric being operable. In general, it is preferred to use the lower pressures, i. e., pressures of less than 50 mm. of mercury, in order to get the reaction products out of the reaction zone as rapidly as possible so that formation of undesirable by-products is minimized. In general, it is also desirable to use the lower pyrolysis temperatures in the above range when operating at the higher pressures, i. e., above 50 mm. of mercury.

The rate at which the fluorocyclobutane is passed through the reaction zone is not critical, although for economic reasons it is preferred to use as high a rate as possible. It is only necessary to heat the tetrafluorocyclobutane to the reaction temperature for a short time to obtain the desired cleavage of the cyclobutane ring. The rate of gas flow through the reactor is greater at the lower pressures. Consequently, the shortest contact times of reactants in the reaction zone are obtained with the lowest operating pressures.

. The reactor can be constructed of any inert, heat-resistant material, e. g., of quartz, heat-resistant glass, stainless steel, or other inert metal. The reactor can, if desired, be packed with inert materials, e. g., granular'quartz, to provide better heat transfer. Metals or other materials which react with the'fluorocyclobutane under the operating conditions to give undesirable by-products should not be used. Inert metals such as'nickel and platinum may be employed. The reaction zone can be heated by conventional means. Electric heaters are very satisfactory for this purpose.

The tetrafluorocyclobutanes used as starting materials in the process of this invention can be prepared by reacting tetrafluoroethylene with a suitable terminally unsaturated ethylenic compound. Such a process is described in U. S. Patents 2,462,345 and 2,462,346, issued to P. L. Barrick.

In the large-scale preparation of the difluoroolefins of this invention, it is possible to carry out the process, including the preparation of the tetrafluorocyclobutane starting materials, by a continuous vapor phase method at atmospheric pressure. In this embodiment the .tetrafluoroethylene and the terminally unsaturated ethylenic compound, e. g., isobutylene, with or without nitrogen as a diluent, are metered into a reaction tube heated to a temperature of 600-1000 C. by an electric furnace. The reaction tube is conveniently made of the heat-resistant glass known commercially as Vycor and is packed with quartz chips to provide better heat transfer.

The invention is further illustrated by the following examples in which the proportions of ingredients are expressed in parts by weight unless otherwise specified.

In these examples the reactor consists of a vertical cylindrical reaction tube approximately one inch in diameter and twelve inches long made of quartz or of a heat-resistant glass, e. g., the type of glass known commercially as Vycor. The reaction tube is packed with 6 mm. sections of quartz tubing 6 mm. in diameter, and is heated externally by means of a cylindrical electric furnace. The temperature of the reaction zone is recorded by a thermocouple placed in the center of the reaction tube. A high capacity vacuum pump maintains the reaction system at the desired reduced pressure. Pressures down to a few microns of mercury are obtained by the use of a mercury diffusion pump. The pressure is measured between the pump and the cold trap in which the products are isolated. This cold trap is cooled by liquid nitrogen. The tetrafluorocyclobutane reactant is introduced into the reaction zone gradually by conventional means, e. g., by means of a dropping tunnel, or by distillation.

EXAMPLE I Five hundred parts of 1-phenyl-2,2,3,3-tetrafluorocyclobutane is passed through a Vycor" reaction tube of the type defined in the previous paragraph maintained at 800 C. and 4-8 mm. pressure. The pyrolysis products are quenched in the trap cooled by liquid nitrogen. After the addition has been completed, approximately 5 hours being required, the trap is removed from the reaction system and the gaseous by-products allowed to evaporate at room temperature. Distillation of the liquid residue gives 109 parts of fi fi'difluorostyrene', B. P. 5859 C./47 mm, and refractive index, n 1.4885. The boiling point and refractive index values for this compound reported in the literature are 65-66'C./6'1-62 mm. and 1x 1.4925. The infrared absorption spectrum obtained on this sample of 5,,8-difluorostyrene is consistent with the structure of this compound. There are also isolated as by-products in this reaction 12 parts of 1,2-difluoronaphthalene or 2,3-difluoronaphthalene, or a mixture of these, melting at 6l-63 C., together with some recovered 1- phenyl'-2,2,3,3-tetrafiuorocyclobutane.

EXAMPLE II EXAMPLE III Using the same equipment as described in Example II, 10 parts of 1,1-dimethyl-2,2,3,3-tetrafluorocyclobutane is passed through a quartz tube held at 825 C. at -30 microns mercury pressure. The reaction products obtained by quenching in a trap cooled by liquid nitrogen, include tetrafiuoroethylene, vinylidene fluoride, isobutylene, and 1,1-difluoroisobutylene.

EXAMPLE IV Using the equipment described in Example II, 800 parts of 1,1-dimethyl-2,2,3,3-tetrafiuorocyclobutane is distilled through the reaction tube maintained at 800 C. and 5 mm. mercury pressure. The reaction products are collected in a cold trap cooled by liquid nitrogen and are then allowed to warm to room temperature. The products which are gaseous at room temperature are distilled through a low temperature still. The major product boils at 7-8 C. and is 1,1-difiuoroisobutylene. The total yield of this material is 140 parts. The molecular weight of a portion of this 1,1-difluoroisobutylene is shown by gas density analysis to be 92 (calculated molecular weight, 92).

Sixty parts of the 1,1-difluoroisobutylene and 60 parts of chlorine are sealed in a Vycor glass reaction tube. On warming to C., the color of chlorine disappears almost immediately. On opening the reaction tube, much hydrogen chloride is evolved. Distillation of the liquid reaction product gives 60 parts of 2-methyl-3- chloro-3,3-difluoropropene-1, boiling at 46.5 C. and having a refractive index n of 1.3580.

Analysis Calculated for C4H5C1F2: C, 38.0%; H, 3.4%; F, 30.0%. Found: C, 38.4%; H, 4.36%; F, 29.5%.

The infrared absorbtion spectrum confirms the structure of this compound as 2-methyl-3-chloro-3,3-difluoropropone-1. Isolation of this product from this chlorination reaction proves the configuration of the 1,1-difloroisobutylene starting material.

EXAMPLE V Using the apparatus described in Example II, 10 parts of 1-methylene-2,2,3,3-tetrafluorocyclobutane is passed through a quartz tube held at 800 C. at a pressure of less than 10 microns of mercury (measured between the cold trap and the pump). The reaction products are condensed in a liquid nitrogen-cooled trap. The reaction product is then allowed to waim to room temperature. The volatile materials contain 1,1-difluoroallene, FzC;=C=CH2, and vinylidene fluoride, CF2=CHa.

Substantially no allene or tetrafiuoroethylene are detected in the volatile reaction product. 1,1-difluoroallene has a very strong absorption at 4.95 microns.

This 1,1-difiuoroallene, a gas at normal temperature and pressure, when passed into a reaction vessel containing bromine cooled below room temperature, is brominated to 1,2,3-tribromo-3,S-difluoro-l-propene, boiling at 58-62 C./2 mm., and having a refractive index n of 1.5410.

Analysis Calculated for CzHFzBrs: C, 11.44%; H, 0.32%; F,

Found: C, 10.04%; H, 0.80%; F, 11.75%; Br, 78.13%.

Infrared absorption spectra and nuclear magnetic resonance measurements are consistent with the proposed structure.

EXAMPLE VI Two hundred and sixty parts of 1-hexyl-2,2,3,3-tetrafiuorocyclobutane is pyrolyzed at 750 C. and 3-13 mm. by the procedure described in the preceding examples. The liquid pyrolysis product is distilled to give 22 parts (volume) of material boiling below 130 C.

Redistillation of this material gives:

Refractive Amount, Fraction B. P., 0. Index, Parts by m Volume below 98 1. 3640 5 98-110 1. 3695 2 110-118 1. 3790 2. 2 118-119 1. 3845 2. l 110 1. 3852 2. 0 119 l. 3853 2. 3 119-120 1. 3857 l. 6

Infrared and elemental analysis indicate that fractions 4, 5, 6, and 7 are principally 1,1-difluorooctene-1.

Analysis Calculated for CzH1-1F4: C, 64.83%; H, 9.52%; F,

25.65%. Found (fraction 6): C, 63.49%; H, 9.59%; F, 24.44%,

EXAMPLE VII Using the equipment and procedure described in the preceding examples 6.5 parts of 1,1,2,2-tetrafluorocyclobutane is pyrolyzed at 800 C. and 1-2 mm. mercury pressure. The reaction products are collected in a cold trap cooled by liquid nitrogen, and are then allowed to Warm to room temperature. The liquid portion is unreacted l,1,2,Z-tetrafiuorocyclobutane and it amounts to about 4.5 parts. The gaseous products, corresponding to about 30% of the starting material, comprise approximately vinylidene fluoride and approximately 15% of equal amounts of ethylene and tetrafiuoroethylene.

The process of this invention has been illustrated in the examples by specific reference to the pyrolysis of 1,1,2,2-tetrafiuorocyclobutane and certain hydrocarbonsubstituted tetrafluorocyclobutanes. However, the invention is generic to the pyrolysis of any 1,1,2,2tetrafluorocyclobutane having a ring CI-Iz group and, on the fourth carbon, i. e., the 3 position of the cyclobutane ring, two hydrogen atoms, an alkylidene or aralkylidene radical of no more than thirteen carbons, or at least one monovalent hydrocarbon radical of up to eight carbons and with a total of thirteen carbons in all hydrocarbon substitue'nts, and free from non-aromatic unsaturation. Specific examples of other such hydrocarbon-substituted tetrafluorocyclobutanes which are operable include 1,1,2,2- tetrafiuorocyclobutanes having in the 3 position either (a) one or two monovalent hydrocarbon radicals free from non aromatic unsaturation whether alkyl, cycloalkyl, aryl or aralkyl, of the above-defined carbon content e. g., tolyl, xylyl, ethylphenyl, ethyl, isobutyl, n-butyl, n-amyl, isoamyl, neopentyl, n-octyl, cyclopentyl, and cyclohexyl groups; or (b) one divalent hydrocarbon radical of up to thirteen carbons whether alkylidene or aralkylidene, e. g., methylene, ethylidene, isopropylidene, benzylidene, and diphenylmethylene groups.

The process of this invention provides an especially convenient synthesis of 1,1-difluoroolefins. The resulting 1,1-difluoroolefins are especially useful as chemical intermediates, for example, for polymerization with peroxy catalyst to polymers useful in impregnating paper to improve the water resistance thereof and for conversion to other fluorine-containing compounds. The ethylenic bonds in these 1,1-difluoroo1efins can be hydrogenated with hydrogen under pressure in the presence of a hydrogenating catalyst to the corresponding saturated difluoro compounds, and they can also be brominated to the corresponding dibromodifluoro compounds. fifi-Difluorostyrene can be reacted with potassium cyanide to form it cyano a fluorostyrene. ,B,18-Difluorostyrene can also be dimerized by heating at 150 C. at several thousand atmospheres pressure. The 1,1-difluoroallene of this invention is particularly useful for reaction with acrylonitrile to form l-methylene-3-cyano-4,4- difluorocyclobutane, and for the formation of novel and useful fluorine-containing polycyclobutane polymers.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described for obvious modifications will occur to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process of obtaining a difluoroolefin which comprises exposing to a temperature within the range 600-1000 C., a tetrafluorocyclobutane of no more than seventeen carbons having two adjacent annular CFz groups, and an annular CH2 group, any substituent on the fourth annular carbon being hydrocarbon of up to eight carbons bonded solely to said fourth annular carbon by from one to two bonds, said hydrocarbon radical being free of non-aromatic unsaturation, i. e., having any unsaturation conjugated in a benzenoid structure, any remaining valence of the fourth annular carbon being satisfied by hydrogen.

2. The process of obtaining a difluoroolefin which comprises exposing to a temperature within the range 750- 900 C., a tetrafluorocyclobutane of no more than seventeen carbons, having two adjacent annular CFz groups and an annular CH2 group, any substitutent on the fourth annular carbon being hydrocarbon of up to eight carbons bonded to said fourth annular carbon by from one to two bonds, the hydrocarbon radical being free of nonaromatic unsaturation, i. e., having any unsaturation conjugated in a benzenoid structure, any remaining valence of the fourth annular carbon being satisfied by hydrogen.

3. The process of obtaining a difluorolefin which comprises exposing to a temperature within the range 750- 900 C., a compound of no more than seventeen carbons and of the formula CFz-CHH Fr-CR wherein R and R are selected from the class consisting of hydrogen and monovalent hydrocarbon radicals of up to eight carbon atoms and free from non-aromatic unsaturation and, when taken together forming a divalent hydrocarbon radical whose two free bonds stem from the same carbon.

4. The process of obtaining a difiuorolefin which comprises passing through a reaction zone maintained at 750900 C., a tetrafluorocyclobutane of not more than seventeen carbons, having the four fluorines attached to two adjacent annular carbons, having on one and only one annular carbon at least one alkyl group of up to eight carbons, all remaining valences of the annular carbons being satisfied by hydrogen.

5. The process of obtaining a difluoroolefin which comprises passing through a reaction zone maintained at 750900 C., a tetrafluorocyclobutane of not more than seventeen carbons, having the four fluorines attached to two adjacent annular carbons, having on one and only one annular carbon at least one aryl group of up to eight carbons, all remaining valences of the annular carbons being satisfied by hydrogen.

6. The process of obtaining a difluoroolefin which comprises passing through a reaction zone maintained at 750-900" C., l-methylene-2,2,3,3-tetrafluorocyclobutane.

7. 1,1-difluoroallene.

8. The process of obtaining vinylidene fluoride which comprises passing l,l,2,Z-tetrafluorocyclobutane through a reaction zone maintained at 750-900 C.

9. The process of obtaining 1,1-difluoroisobutylene which comprises passing 1,l-dirnethyl-2,2,3,3-tetrafluorocyclobutane through a reaction zone maintained at 750- 900 C.

10. The process of obtaining 1,1-difiuoropropylene which comprises passing 1-methyl-2,2,3,3-tetrafiuorocyclobutane through a reaction zone maintained at 750900 C References Cited in the file of this patent UNITED STATES PATENTS 2,617,836 Pearlson Nov. 11, 1952 

1. THE PROCESS OF OBTAINING A DIFLUOROOLEFIN WHICH COMPRISES EXPOSING TO A TEMPERATURE WITHIN THE RANGE 600-1000* C., A TETRAFLUOROCYCLOBUTANE OF NO MORE THAN SEVENTEEN CARBONS HAVING TWO ADJACENT ANNULAR CF2 GROUPS, AND AN ANNULAR CH2 GROUP, ANY SUBSTITUENT ON THE FOURTH ANNULAR CARBON BEING HYDROCARBON OF UP TO EIGHT CARBONS BONDED SOLELY TO SAID FOURTH ANNULAR CARBON BY FROM ONE TO TWO BONDS, SAID HYDROCARBON RADICAL BEING FREE OF NON-AROMATIC UNSATURATION, I.E., HAVING ANY UNSATURATION CONJUGATED IN A BENZENOID STRUCTURE, ANY REMAINING VALENCE OF THE FOURTH ANNULAR CARBON BEING SATISFIED BY HYDROGEN. 